ASDEX Upgrade banishes instabilities

Theory for the ITER international experimental reactor verified / obstacle to power plant eliminated

The objective of the world-wide efforts in nuclear fusion is to develop a power plant that, like the sun, derives energy from fusion of atomic nuclei. To ignite the fusion fire the hydrogen plasma fuel has to be heated to temperatures of more than 100 million degrees. In order to maintain such high temperatures, it is necessary to confine the fuel in magnetic fields to prevent contact with the vessel walls and thermally insulate it from them. The complex interaction between plasma particles and the magnetic cage, however, makes a whole series of instabilities possible, thus impairing confinement. Particularly undesirable are so-called neoclassical tearing modes: They occur when the temperature and pressure of the plasma approach the ignition values.

Just how dangerous these instabilities can be to the performance of the planned ITER international test reactor was already predicted six years ago by plasma theoreticians at IPP: They calculated the upper limit for the plasma pressure to be lower the larger the device – in ITER ten times as low as in the smaller ASDEX Upgrade; that is, a major difficulty for ITER and a serious obstacle on the way to an economically operating power plant. A European group headed by IPP was therefore formed to tackle this problem for ITER. It comprises scientists from the University of Stuttgart and from the fusion centres in the UK, the Netherlands and Italy.

Before the tearing modes can be combatted it first has to be established why they happen: In building the magnetic field cage for the plasma, fusion researchers utilise the fact that the charged plasma particles – ions and electrons – are compelled by electromagnetic forces to move on helical paths about magnetic field lines. Guided by an appropriately shaped magnetic field as on rails, the fast particles can thus be kept away from the walls of the plasma vessel. To make a cage “tight” the field lines have to form inside the ring-shaped plasma vessel closed surfaces nesting inside one another – like the growth rings of a tree trunk. This prevents field components directed outwards, which would lead the plasma particles to the walls. The high ignition temperatures needed would then be unattainable. On each magnetic surface the density and temperature are constant, whereas from surface to surface – from the hot centre outwards – the density, temperature and plasma pressure decrease.

So much for the principle – were it not for the instabilities that deform the confining magnetic field. As exact analysis shows, the previously symmetric plasma ring is subject to blister-like perturbations with their own magnetic field structure closed upon itself: magnetic “islands”. These are triggered as the plasma pressure rises with increasing plasma temperature. With the emergence of the islands the magnetic field lines tear and link up with the field lines of neighbouring magnetic surfaces. This gives rise, as it were, to magnetic short-circuiting. Since now fast energy exchange can also occur transversally to the surfaces, the plasma temperature and plasma pressure drop right across the island. This restricts the plasma pressure attainable: The power yield of ITER and a future power plant would thus be greatly impaired.

As the upper limit for the plasma pressure is the lower the bigger the fusion device, tearing modes at first seemed inevitable in a power plant. All the greater was the stir created when the ASDEX Upgrade team succeeded in 1999 for the first time in hampering the formation of such magnetic islands: This was done by accurately beaming microwaves – within centimetres – into the centre of an emerging island. This generates a local electric current that makes the island vanish. The magnetic field perturbation is suppressed and the plasma pressure can rise again. A resounding success was then achieved a year later when it became possible to eradicate an island completely. The new method was verified soon thereafter in fusion devices in the USA and Japan.

As scientists from IPP were able to report at the conference, not only has it been possible in ASDEX Upgrade to stabilise a particularly disturbing tearing mode that can lead to disruption of a discharge; after improvement of the beaming method this could also be achieved with a much lower microwave power: For stabilisation less than ten per cent of the total heating power applied was sufficient – when precisely beamed to the right spot. Professor Dr. Hartmut Zohm of the ASDEX Upgrade team: “We are sure that an instrument for controlling magnetic islands has been found. We must now investigate whether it is routinely suitable for ITER.”

To take this step from physics to technology the method is to be automated: Detection of islands and microwave beaming are to be incorporated in the automated feedback control of ASDEX Upgrade. The system is to record independently the formation of an island, then sight it with moving mirrors, and trigger the microwave beam. A controllable microwave coupling system for this purpose is already envisaged in the planning for ITER.

Isabella Milch